In this work, Ni/C/N nanocomposites were synthesized from wheat straw and sugar beet pulp by two-stage direct pyrolysis using nickel chloride and urea as activating agents. It was demonstrated that nitrogen doping of the precursor, achieved using urea as the nitrogen source, induces ferromagnetic behavior in the carbon material. Upon additional activation with nickel chloride, the magnetic behavior of the C/Ni/N composites is governed by both Ni nanoparticles with an average size of approximately 100 nm and the magnetism of the nitrogen-doped carbon matrix. Wheat-straw-derived composites exhibited predominantly microporous structures with larger BET surface areas, whereas sugar-beet-pulp-derived samples contained a higher proportion of mesopores. Although the latter possessed lower specific surface areas, they demonstrated superior adsorption capacities toward Congo Red owing to improved accessibility of adsorption sites for large dye molecules. The adsorption process was best described by the Langmuir model, while the Dubinin–Radushkevich model suggested strong specific physicochemical interactions between dye molecules and the adsorbent surface. The results demonstrate that the adsorption efficiency of biomass-derived Ni/C/N nanocomposites is governed not only by the total surface area but also by pore architecture and surface chemistry.